The theory of plate tectonics is only a few decades old, meaning that a great many geologists working prior to its discovery had much more trouble understanding the landscape around them than we do today. Some of the ideas that popped up to fill this vacuum can seem peculiar to us now.

Take, for example, the “contracting Earth” hypothesis championed in the 1800s by James Dwight Dana and also proposed by Élie de Beaumont. They sought to make sense of mountain ranges, faults, and bending folds of rock with an appeal to Earth’s thermal history. The Earth had likely cooled from an initial molten state, they reasoned, and should therefore also have contracted in size. The outer crust of the Earth, exposed as it is, would have cooled first. As the hotter interior continued cooling and shrinking, the already-solid crust would have to crinkle, crack, and buckle in response—hence the faults and mountain ranges.

While this turned out to be the wrong explanation for the geology on Earth, it was rescued from the bin of discarded hypotheses by other bodies in the Solar System.

The planet Mercury, which is much smaller than the Earth, doesn’t have tectonic plates (plural)—it has one tectonic plate. Despite this geological unity, there are linear features on its surface caused by the squeezing of the rock. Without colliding tectonic plates to provide that squeezing, you need another mechanism. For Mercury, contraction fits the bill. Because it’s much smaller than the Earth, it cooled much more quickly, leading to a very different style of tectonics.

So how much has it shrunk? Planetary geologists have been trying to figure that out for a while. Models of the cooling process predict that the planet’s radius should have decreased by 5 to 10 kilometers, a reduction of about 0.2 to 0.4 percent. Using imagery from NASA’s Mariner 10 mission in 1974, however, researchers only saw evidence of surface wrinkles to corroborate 0.8 to 3 kilometers of radial shrinkage.

Of course, the Mariner 10 data wasn’t perfect—an entire hemisphere of Mercury wasn’t imaged—and this estimate was never taken as the final word. The spectacular data from the recent MESSENGER mission provided an opportunity to revisit the question, and Paul Byrne of the Universities Space Research Association headed a team that took full advantage of it.

Using MESSENGER’s planet-wide coverage and topographic data, the researchers mapped almost 6,000 tectonic ridges. These can run anywhere from 9 and 900 kilometers across Mercury’s surface, rising several hundred meters or more into its very thin atmosphere. The surface rocks aren’t actually broken by faults. Rather, they’re continuous layers draped over faults beneath the surface.

Picture a short stack of printer paper sitting on top of two wooden blocks. If you push one block upward (as one side of a faulted rock can move), the paper will bend in response, creating a muted version of the sharp ledge between the blocks. Mercury’s ridges are akin to this sort of upheaval.

The ridges aren’t evenly distributed around the planet. A region of volcanic plains in the northern hemisphere that encompasses about 6 percent of Mercury’s surface contains about 28 percent of its tectonic ridges. There are also some bands of closely spaced ridges that may represent networks of related faults common in mountainous areas on Earth.

By analyzing all these ridges, the researchers were able to calculate a new estimate for the contraction that created them—a reduction of Mercury’s radius by about 5 to 7 kilometers. That’s much higher than the older estimates, and it’s in line with the predictions of the mathematical models.

The interest in this area goes beyond getting the number right. That bit of information helps us sort out more details about the planet closest the Sun—it puts limits on the amount of radioactive isotopes that could have been present in early Mercury to generate heat, for example. It’s helpful for working out whether there’s any convection in Mercury’s mantle today and how its magnetic-field-producing metallic core cooled over time. We can also use it to make other inferences about Mercury’s surface. There’s still a lot we don’t know about the planet that shrank.